Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $116.67   3.337%   $120.5633   $120.56   $120.55   $120.60  
  Purchase #2 $20.59   13.732%   $23.4174   $23.42   $23.40   $23.40  
  Purchase #3 $212.80   4.617%   $222.625   $222.63   $222.65   $222.60  
  Purchase #4 $203.31   3.190%   $209.7956   $209.80   $209.80   $209.80  
  Purchase #5 $55.58   6.347%   $59.1077   $59.11   $59.10   $59.10  
  Purchase #6 $218.99   5.169%   $230.3096   $230.31   $230.30   $230.30  
  Purchase #7 $61.60   6.777%   $65.7746   $65.77   $65.75   $65.80  
  7 purchase totals = $889.54       $931.5932   $931.60   $931.55   $931.60